In an ideal situation, sleep should be undisturbed and experienced in the same room every night

Sleep is the regular state of natural rest observed in all mammals, birds and fish. Sleep is not actually "unconsciousness," but rather, it is a natural state of rest characterized by a reduction in voluntary body movement and decreased awareness of the surroundings. Therefore, since consciousness is literally the awareness of the surroundings, being asleep is just an altered state of consciousness, as opposed to being unconscious. It is heavily influenced by circadian rhythms, and by hormonal and environmental factors as well. Sleep appears to perform a restorative function for the brain and body, as evidenced by the myriad symptoms of metabolic dysfunction that result when animals are deprived of sleep (Gottlieb et al., 2005).

The function of sleep in health and in disease is being increasingly studied in specialized sleep laboratories throughout the world. Not only insomnia, but more recently elucidated sleep disorders such as sleep apnea and narcolepsy are evaluated in such facilities.
The increasing prevalence of sleep disorders is likely to be a function both of more sophisticated diagnostic tests and the disruption of the normal day-night cycle in modern societies.

Sleep is often defined using specific criteria relating to EEG data. All mammals and birds fulfill the criteria for sleep based on EEG recordings. In animals where EEG data is not readily available, or their small size precludes recording an EEG, behavioral and gene specific data are utilized for sleep studies.

The cycle of sleep and wakefulness is regulated by the brain stem, external stimuli, and various hormones produced by the hypothalamus. Some neurohormones and neurotransmitters are highly correlated with sleep and wake states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleoside involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep. Researchers believe that its accumulation during the day encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in the regulation of circadian rhythms. The SCN is influenced by external light and also generates its own rhythm in isolation. In the presence of light it sends messages to the pineal gland that instruct it to cease secreting melatonin.

Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage is sometimes referred to as somnolence, or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2), and can be associated with so-called hypnagogic hallucinations. In this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep.

Stage 2, with "sleep spindles" (12–16 Hz) and "K-complexes". The EMG lowers, and conscious awareness of the external environment all but disappears. This occupies 45-55% of total sleep.

Stage 3, with delta waves, also called delta rhythms (1–2 Hz) is considered part of SWS and functions primarily as a transition into stage four. Overall it occupies 3-8% of total sleep time.

Stage 4 is true delta sleep. It predominates the first third of the night and accounts for 10-15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors and sleepwalking occur.

Stage 5, or Rapid eye movement (REM) sleep, associated with dreaming, especially bizarre, visual, and seemingly random dreams. REM sleep is predominant in the final third of a sleep period, its timing linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves.

Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see sleep deprivation below). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

Restorative theories of sleep describe sleep as a dynamic time of healing and growth for organisms. For example, during stages 3 and 4, or slow wave sleep, growth hormone levels increase, and changes in immune function occur. The myriad illnesses associated with sleep deprivation testify to its restorative function.

According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or Active Sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004). Given sleep's heterogeneous nature, however, no single theory predominates, as it is difficult to describe one single "function" of sleep.

One process known to be highly dependent on sleep is memory. REM sleep appears to help with the consolidation of spatial and procedural memory, while Slow-wave sleep helps with the consolidation of declarative memories. When experimental subjects are given academic material to learn, especially if it involves organized, systematic thought, their retention is markedly increased after a night's sleep. Mere rote memorization is retained similarly well with or without an intervening period of sleep.

Non-REM sleep is an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems. Sleep also restores neurons and increases production of brain proteins and certain hormones. Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates. Asking the question "Why do we awaken?" instead of "Why do we sleep?" yields a different perspective toward understanding how sleep and its stages contribute to a healthy organism.

The demonstrably necessary phenomenon of dreaming would suffice to prove the importance of sleep to humans, and perhaps to other animals as well. Dreaming involves an involuntary conjuring up of sometimes magical images in a story-like sequence in which the sleeper/dreamer is usually more a participant than an observer. Most scientists agree that dreaming is stimulated by the pons and occurs during the REM phase of sleep.

Many functions have been hypothesized for dreaming. Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconsciousness, and used dream interpretation in the form of psychoanalysis he pioneered. Scientists today have become more skeptical about details of Freudian interpretation, and place more emphasis on dreaming as a requirement for organization and consolidation of recent memory and experience.
Another theory is that dreaming allows an animal to play out scenarios that may help the animal avoid dangers when awake. For example, a rabbit might dream about being cornered by a lion and may play out different scenarios that might increase chances of survival should he come across a lion in reality.

A common misperception is that everyone needs eight hours of sleep. The amount of sleep needed is different for each person. This amount needed is individually and biologically determined. Some can do with six hours of sleep, others need nine. However, as a general rule, eight hours is recommended. Sleep experts state that you cannot "store" sleep by sleeping more on the weekends in preparation for the normal work week. Just how much sleep deprivation leads to death in humans is unknown.[1]

Another commonly held view is that the amount of sleep one requires decreases as one ages, but this is not necessarily the case. The ability to sleep, rather than the need for sleep, appears to decrease when people get older. [2]

Many sleep disorders result from errors in synchronization of sleep with the body clock. Other sleep problems are organic and cannot be resolved with chronotherapy. One often effective solution to some kinds of insomnia involves free-running sleep. Free-running sleep entails ignoring alarm clocks and schedules in order to sleep when, and only when, tired. Free-running sleep can resolve the majority of synchronization-dependent sleep disorders, but is difficult to sustain due to the resulting loss of synchronization of sleep with the outside world (including the day-night cycle).

Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for bats. Generally, required sleeping time decreases as body size increases. Cats are one of the few animals that do not have most of their sleep consolidated into one session, preferring instead to spread their sleep fairly evenly throughout the day.

Water mammals "sleep" with alternate hemispheres of their brains asleep and the other awake. They need to do this so they can breathe above water while sleeping. Migratory birds also seem to sleep this way.

Even fish and fruit flies appear to have a "sleeplike" state. This alternation of the sleeplike state and its absence is referred to as a "Basic Rest and Activity Cycle", or BRAC. Since the modern definition of sleep is defined using EEG criteria, and such tiny brains preclude the recording of EEG's, this may not technically be described as sleep. However, if fruit flies are repeatedly disturbed so that they can not rest, they have what is referred to as a "rest rebound". This behavior is strikingly similar to that exhibited by mammals and birds in similar conditions. As research equipment improves, the definition of sleep may soon be revised.

Many animals hibernate in a deep sleeplike state during winter to conserve body heat and energy. Estivation is a similar state in which other animals hibernate to escape the heat of summer.

Cattle, horses, and sheep are unique in that they can sleep while standing, though for cattle and sheep, REM sleep will not occur in such a position. For REM sleep to take place, the animals must lie down. Sleeping while standing is thus only partial sleep. However, birds may have periods of REM sleep while perched.

Many non-pharmacological approaches exist to improve falling asleep and staying asleep. Doctors and health professionals may suggest any of the following, depending on the type of sleep disruption, the person's situation, and their specific sleep needs.

Decrease the light level in the sleeping environment. Studies have indicated that the brain has a separate neural pathway to the optical nerve, separate from the visual path, to detect whether it's day or night. This detection system could have a direct effect on successful sleep inducement. Moreover, other studies have shown sleep inducement is dramatically increased by reductions in light level, in the sleeping environment. Use appropriate curtains and shading to keep light out or at minimal levels.

Set a quiet time approximately 30 minutes before bedtime—no computer, television, video games, office work, housework, or other stressful, dutiful, or mentally stimulating activities—to slow down the metabolic rate.

Reading or other light mental activity at bedtime.

Milk contains tryptophan, which may help relax the nervous system and induce drowsiness. However, this theory is likely rooted in a folk remedy, and scientific evidence suggests that simply drinking milk may or may not help.

Get up to do some quiet activity or slowly walk around until feeling tired, if one does not fall asleep in bed after 20 to 30 minutes. Paradoxically, this may increase restlessness in some people.

Make sure one's sleeping position or posture is comfortable and provides enough support, especially for the lower back.

Quiet, slow-paced, simplistic music can also help sleep inducement. Audio music products are available on the market that are designed to help fulfill this function.

Avoid using the bed for activities other than sleep, to maintain an association between getting into bed and sleeping.

Avoid certain drugs (e.g., cocaine and Ritalin), which are stimulants and can adversely affect sleep. Many non-stimulant medications, including glucocorticoids, may also cause sleeping problems.

The pharmacological approach to inducing sleep involves the use of depressant drugs [4], formerly barbiturates, but today usually benzodiazepines, which depress the central nervous system (the brain and spinal cord). Non-prescription antihistamines, which are commonly marketed as sleep aids, are also available. Today, sleeping pills are generally prescribed only on a temporary basis and only if symptoms of insomnia are severe enough to seriously impede a patients life. It is possible habitual consumption of depressants to aid sleep leads to drug dependence. As with most depressents, drug tolerance may develop, as well as many long term side effects; it can cause paradoxical insomnia.

Conversely, drugs may also be used to dissuade or delay sleep. The stimulantmodafinil reduces drowsiness and is prescribed to treat narcolepsy. It allows the user to cope with shorter sleep periods while producing only minimal euphoric effects. The amphetamine family of stimulants by comparison would help to keep people awake longer but preclude the opportunity for a given person to obtain "recovery sleep" for a longer time. Modafinil, while maintaining a longer elimination half-life, does seem to allow sleep earlier than amphetamines. Simply put, modafinil removes the urge to fall asleep, rather than encouraging wakefulness. Modafinil also appears to reduce, but not elimate, the need for recovery sleep, compared to amphetamine and placebo PMID 10607162.